1. Field of the Invention
The present invention relates to a reactance control circuit, and more particularly to a reactance control circuit for use in a variable frequency oscillator.
2. Description of the Related Art
An example of a typical conventional variable frequency oscillator is shown in FIG. 1, and comprises a differential amplifier 2, a DC amplifier 3, a variable reactance circuit 4, and an oscillator 5. The oscillating frequency generated by the oscillator 5 is determined by an oscillating frequency of the oscillator 5 and the value of reactance of the reactance circuit 4. The reactance circuit 4 is made up of a capacitor C1, a resistor R5, and differential amplifiers 6 and 7, respectively coupled with the oscillator 5 in a negative feedback and a positive feedback fashion. Since the negative feedback loop has the effect of decreasing the output frequency of the oscillator 5, and the positive feedback loop increases the same, therefore the reactance value of the variable reactance circuit 4 can be varied in accordance with the ratio (I3/I4) of the value of a current I3 flowing through the differential transistor circuit 6 and that of a current I4 flowing through the differential transistor circuit 7. When the current I3 is greater than the current I4, the output frequency of the oscillator 5 is decreased by the negative feedback loop of the differential transistor circuit 6, with the result that the reactance value C of the variable reactance circuit 4 increases. When on the other hand, the current I3 is less than the current I4, the output frequency of the oscillator 5 is increased by the positive feedback loop of the differential transistor circuit 7, and thus the reactance value C of circuit 4 decreases.
The current ratio I3/I4 is determined by the differential amplifier 2 and the DC amplifier 3. In the differential amplifier 2, currents I1 and I2 are determined by the difference in the level of an input voltage Vin and a reference voltage Vref1, are amplified by the DC amplifier 3, and supplied in the form of currents I3 and I4 to the differential transistor circuits 6 and 7.
When there is zero difference in the level of the input voltage Vin and the reference voltage Vref1, the currents I3 and I4 are then equal, in which case the negative feedback operation by the differential transistor circuit 6 and the positive feedback operation by the differential transistor circuit 7 cancel each other out. Therefore, the reactance value of the variable reactance circuit 4 is determined solely by capacitor C1 and the oscillator 5, independent of the currents I3 and I4, and the reactance value is equal to a reference reactance Cref. When the reactance value is equal to the reference reactance Cref, the variable frequency oscillator generates an output signal Vout of a free running frequency Fref.
The DC amplifier 3 of the variable frequency oscillator shown in FIG. 1 uses NPN transistors Q1 and Q2. However, the geometrics and operating characteristics of such transistors are frequently nonuniform, this being attributable to the process used to manufacture them. Thus, when there is zero difference in the level of the input voltage Vin and the reference voltage Vref1, and hence the current I1 is equal to the current I2, the reactance value C of the reactance circuit 4 may not be equal to the reference reactance Cref, for the reason that even if the currents I1 and I2 applied to the bases of the transistors Q1 and Q2 are equal to each other, the currents I3 and I4 passing through the transistors Q1 and Q2 may not be equal if the operating characteristics and geometrics of these transistors differ from each other. Consequently, if the reactance value C of the variable reactance circuit 4 is not equal to the reference reactance value Cref, the free running frequency Fref of the variable frequency oscillator will differ from its designed value.